In packet-based networks, it is often desired to test communications between two specific nodes on the network. This can generally be effected from a first one of the nodes by requesting the other node to `loop-back` a test packet sent from the first node. The first node, on receiving back the test packet, can thereby ascertain not only that communication is possible with the other node, but also the round trip time for the packet. Typically such a test would be performed by an instrument such as the HP 4982A LAN protocol analyzer (available from Hewlett-Packard Company, Palo Alto, Calif.) connected to the network at the first node.
Inter-networking protocols such as the ARPA Internet Protocol (IP) may also provide a facility for determining reachability and round trip time by use of a looped-back test packet. Thus, in IP networks the Internet Control Message Protocol CiCMP) allows control and information messages, including echo request and echo reply messages, to be passed in the data portion of IP datagrams between IP software on different hosts and gateways. Other messages allowed by ICMP include timestamp request and timestamp reply messages which permit a transit time estimate to be made in both transit directions. Certain computer operating systems such as the Hewlett-Packard HPUX operating system permit users to send ICMP echo requests using a command named "ping" with the users being able to specify the number and size of test packets for which round trip times are returned.
It may also be noted that in the ARPA Transmission Control Protocol (TCP), a similar measurement is continually made as part of the transmission control process. More particularly, the round-trip time is measured between the transmission of a packet and receipt back of an acknowledgement from the destination node; this round-trip time is averaged continually into a smoothed round-trip time estimate which is then used to control the retransmissions time-out parameter RTO.
The use of looped-back test packets has thus been restricted to determining the directly-observable round-trip time characteristic of the transmission path between two nodes.
It is an object of the present invention to permit further network characteristics to be determined from the operation of transmitting packets between two nodes, these characteristics being those which are not discernible from the transmission of a single packet between the nodes.